Over the past 4 years, considerable data has indicated that insulin increases skeletal muscle blood flow in healthy humans and insulin- resistance in NIDDM and obese skeletal muscle is accompanied by a diminished vascular response. It is not known whether the increase in muscle blood flow is due to enhanced flow to previously perfused muscle or to recruitment of new capillary beds. Using two techniques developed in our laboratories, we will test the hypothesis that insulin and IGF-I act to open new capillary beds. These studies are ongoing. Preliminary results indicate that insulin not only induces this changes in total skeletal muscle blood flow, but also recruits new capillary elements thus affecting blood flow distribution within muscle. Future experiments will take advantage of albumin coated microbubbles that can be injected intravenously rather than interarterially.